What does "the difference between a treatment group and a control group is of 0.2 standard deviations" mean?

I'm reading a research paper but can't remember whether the standard deviation relates to the mean of each group or to something else? How would you rephrase this statement?|||The standard deviation is a measure of how "spread out" the data is, i.e. a measure of how far data is from the mean, on average. Well actually there is a square involved. To be precise, if there are n data points, x1, x2 all the way to xn, then:



s.d. = sqrt root of ( [ (x1-mean)^2 + (x2-mean)^2 + ... + (xn-mean)^2] / n)

..........( NOTE CORRECTION "/n" not "/2")

= sqrt root of (average (distance squared) from the mean)



The standard deviation has the same units as the quantity being measured. So if it's a set of women's weights, the standard deviation might be 10 kg, meaning "0.2 standard deviation" is 0.2 times 10 kg = 2 kg,



EDIT: Corrected a silly typo